Well cleanout tools find their maximum utility when employed for removing frac sand from the bottom of the borehole. From time to time, it is necessary to acidize a formation by pumping acid downhole through the perforations in the casing and back up into the hydrocarbon producing formation. Propping agents, such as specially selected sand, are pumped downhole either during or after the acidizing operation. After the fracturing operation has been completed, the borehole will often contain 100 feet or more of sand which lies in proximity of the perforated zone, and accordingly, the sand must be removed before the well is placed back onto production.
Well cleanout tools utilizing hydrostatic pressure for inducing flow may be actuated by a sand line, or the tool may be tubing conveyed. The debris capacity of the tool depends upon the size of the debris-containing reservoir coupled with the magnitude of the hydrostatic head, both of which must be considerably large if a significant quantity of debris is to be recovered.
It is expensive to charge a borehole with several truckloads of brine in order to achieve the hydrostatic head required of prior art hydrostatic bailers. It is expensive to build and operate a tool having a tremendous debris-containing reservoir. It is also expensive, especially when the tool is tubing conveyed, to make a plurality of trips into the borehole in order to retrieve all of the junk located downhole therein.
Running a tool of the above type is dangerous because the recovered tool must inherently have an internal pressure of more than 1000 psi sometimes. It is, therefore, possible to cause fatal or serious injuries when the tool is subsequently disassembled.
Occasionally, the tailpipe of a well cleanout tool becomes submerged into the debris to such an extent that the tool becomes lodged in the borehole. Such a disastrous situation calls for a workover rig, pumps, and an overshot in order to wash the debris from the tool, thereby freeing the tool after spending a substantial amount of money on a "fishing job".
Accordingly, it would be desirable to obviate the above mentioned drawbacks by the provision of an improved wellbore cleanout tool. Such a desirable expedient is the subject of the present invention.